1. Field of the Invention
One embodiment of the invention relates to a modulator that uses code-division multiplexing, a radio transmitter that uses the modulator, and a radio receiver that receives a signal transmitted from the radio transmitter.
2. Description of the Related Art
It is possible to separate, on a reception side, a signal modulated in the code-division multiplexing into plural user signals and also separate the signal into plural paths. Therefore, a method called Rake combination for performing equalization processing by combining times and phases of separated paths is often used. It is known that, in the Rake combination, since the number of separable paths is limited due to hardware limitation, a reception characteristic is deteriorated when paths exceeding the limited number arrive on the reception side.
On the other hand, there is known a method of transmitting a signal modulated in the code-division multiplexing with a guard interval (GI) added thereto, converting the signal into a signal of frequency domain according to Fast Fourier Transform on a reception side, and performing equalization using the frequency domain. In this method, it is known that, since it is possible to combine paths having delays shorter than the period of the guard interval at a time, the reception characteristic is better than that in the Rake combination when a large number of paths arrive on the reception side (see “Chip Interleaving for DS-CDMA with Frequency Domain Equalization in a frequency selective Rayleigh fading channel”, Garg Deepshikha, Fumiyuki Adachi, IEICE Tech. Rep, RCS2003-304, March 2004, p. 7 to 12).
However, the method of performing equalization using the frequency domain has a disadvantage in that overheads are caused by the added guard interval and throughput decreases when no error occurs. Therefore, it is desirable to perform equalization in frequency domain by adding the guard interval to a signal when the number of paths is large and to perform the Rake combination (i.e., equalization in time domain) without adding the guard interval to the signal when the number of paths is small. However, if the guard interval is added to signals for some users and is not added to signals for other users, presence and absence of the GI are mixed among the users. When signals for plural users are multiplexed in a base station apparatus according to the code-division multiplexing, orthogonality of codes is broken, which deteriorates the reception characteristic. Thus, it is necessary to switch presence and absence of the guard interval collectively for all the users. On the other hand, if the guard interval is added to signals for all the users, overheads simply increase for users who receive signals using the Rake combination. As a result, throughput is deteriorated.